Heat-sensitive recording materials produce a recorded image by using the physical or chemical change of a substance due to thermal energy, and many thermographic processes have been studied. "Wax type" heat-sensitive recording sheets that use the physical change of a substance due to thermal energy have been known for many years, and are used, e.g., in electrocardiograms. A number of heat-sensitive recording sheets that depend on various color forming mechanisms using the chemical change of a substance due to thermal energy have been proposed, and "two-color forming component" heat-sensitive recording sheets are typical of this type of sheet.
To make a two-color forming component heat-sensitive recording sheet, typically a dispersion of fine particles of two thermal reactive compounds is mixed with binder particles to form a coating solution, which is applied to a base in such a manner that the two thermal reactive compounds remain separated from each other by the binder particles. This sheet forms an image by using a color forming reaction that is initiated upon contact between the two compounds when one or both compounds are melted with heat. The two compounds are generally referred to as an electron donating compound and an electron accepting compound, respectively. While there are many combinations of these compounds, they generally consist of those which form an image of metallic compounds, and those which form a dye image. Examples of the combinations that form a dye image typically use electron donating colorless dyes as the electron donating compound and phenols and other acidic substances as the electron accepting organic compound (Japanese Patent Publication Nos. 4160/68 and 3680/69).
These two-color forming component heat-sensitive recording sheets have advantages for use as a heat-sensitive recording sheet, e.g., (1) they depend on a primary color forming reaction, and require no developing step, (2) the texture of the sheet is similar to that of ordinary paper, and (3) they are easy to handle. The sheets using colorless dyes as the electron donating compound have greater commercial value in that they achieve not only the three advantages described above but also the following two additional merits: (4) they provide a higher color density, and (5) heat-sensitive recording sheets providing various colors can be easily prepared. In view of these advantages, the latter type of sheets are used most frequently as heat-sensitive recording materials, e.g., for image recording sheets for facsimiles. The heat-sensitive recording materials of dye image forming type are also used in an increasing volume as data recording means to be incorporated in laboratory equipment, medical equipment, and industrial measuring instruments.
The most common heat-sensitive recording material that is used currently is a heat-sensitive recording paper having a paper base. Paper is used as the base material for various reasons--low price, stiffness, and compatibility with writing devices. Therefore, the following description of the process and features of the present invention is directed to the most commonly used heat-sensitive recording paper, but it should be understood that the present invention is by no means limited to this particular type of heat-sensitive recording material.
As was already mentioned, the heat-sensitive recording paper is used for recording facsimile and other forms of information. To be used in many applications, heat-sensitive recording papers having various thermographic characteristics must be developed. For two- or three-color formation, a recording material comprising a plurality of layers that produce different colors (e.g., a two-layer arrangement wherein the upper layer contains a color former that forms a blue color at lower temperatures and the lower layer contains a color former that forms a red color at higher temperatures) is scanned with a hot pen at different temperatures. In this color forming process, the respective layers must have different thermal sensitivities for color formation. For recording in facsimiles, several types of recording paper that have mode-adapted sensitivities are necessary, since the recording speed varies with the operation mode.
One example of the heat-sensitive recording paper is a paper base coated with a dispersion of an electron donating colorless dye (hereunder referred to as a color former) and an electron accepting organic compound (hereunder referred to as a color developer) in a binder made of a water-soluble polymer. When the paper is heated, the color developer or color former is melted and spread through the heat-sensitive color-forming layer to contact the color former or color developer to cause a color forming reaction and provide a desired recorded image. The temperature dependency of the sensitivity and image density of the heat-sensitive recording paper varies with the temperature at which the color developer (or color former) is melted and their color forming properties. The characteristic curves for three heat-sensitive recording systems are depicted in FIG. 1 wherein curve A shows the highest heat-sensitivity. The recording systems having curves B and C exhibit different behaviors at about 200.degree. C. which is the operating temperature of the heating element. The system having curve A provides gradation over a wide range of color-forming temperatures, and is adapted to record an image requiring a wide gradation in density. On the other hand, a multi-layer heat-sensitive recording paper using heat-sensitive color-forming layers having two curves, say, curves A and B, can be used to record information in the form of a two-color image.
In the production of a heat-sensitive recording paper, it has been found that a color can be formed at a desired temperature between ordinary temperatures (i.e., 20.degree. C. to 30.degree. C.) and the melting point of the developer by incorporating in the heat-sensitive color-forming layer (1) fine particles of eutectic mixture crystals prepared by melting an electron accepting organic compound (color developer) with one or more organic compounds that change its melting point, or (2) fine particles of a mix of the eutectic mixture crystals of color developer and melting point modifier and the crystals of the respective components, or (3) fine particles of a solid solution of the developer and melting point modifier, or the combination of a core made of one component crystal (e.g., fine crystal of color developer) and a skin made of non-crystalline or partially crystalline form of the other component (e.g., melting point modifier), for example, as described in Japanese patent application No. 110942/80, or (4) fine particles obtained by grinding a non-crystalline solid cooled from a melt of the developer and melting point modifier (the melt may crystalize if the melt is maintained for a long period of time). In short, by using these fine particles, heat-sensitive recording papers capable of changing image density over a wide range of developing temperatures (those papers capable of providing a wide gradation) can be designed.
Amides of long-chain aliphatic carboxylic acids and primary amines are known as substanes that can be used with the electron accepting organic compound (color developer) to change its melting point; see Japanese patent application (OPI) No. 48751/78 (the term OPI as used herein means an unexamined published Japanese patent application). However, such amides can vary the melting point of the color developer only over a very small range, and this is particularly so if they are contained in fine particles of organic compounds having a phenolic hydroxyl group, aromatic or aliphatic organic carboxylic acids, salicylic acid and salicylic acid derivatives as described in Japanese patent application (OPI) No. 48751/78. Among the compounds mentioned in Japanese patent application (OPI) No. 48751/78, stearic acid amide, oleic acid amide, palmitic acid amide, and lauric acid amide can hardly change the melting point of the developers as described above.